1. Field of the Invention
The present invention relates to an electrochemical device having an anode containing magnesium, a cathode stable to a voltage of at least 3.2 V relative to a magnesium reference; and an electrolyte magnesium salt obtained by mixing a magnesium salt with an aluminum compound in an ether solvent. Most specifically the invention is directed to a magnesium sulfur electrochemical device having a stable and safe electrolyte which is compatible with a magnesium anode and a sulfur cathode.
2. Discussion of the Background
Lithium ion batteries have been in commercial use since 1991 and have been conventionally used as power sources for portable electronic devices. The technology associated with the construction and composition of the lithium ion battery (LIB) has been the subject of investigation and improvement and has matured to an extent where a state of art LIB battery is reported to have up to 700 Wh/L of energy density. However, even the most advanced LIB technology is not considered to be viable as a power source capable to meet the demands for a commercial electric vehicle (EV) in the future. For example, for a 300 mile range EV to have a power train equivalent to current conventional internal combustion engine vehicles, an EV battery pack having an energy density of approximately 2000 Wh/L is required. As this energy density is close to the theoretical limit of a lithium ion active material, technologies which can offer battery systems of higher energy density are under investigation.
Magnesium as a multivalent ion is an attractive alternate electrode material to lithium, which can potentially provide very high volumetric energy density. It has a highly negative standard potential of −2.375V vs. RHE, a low equivalent weight of 12.15 g/mole of electrons and a high melting point of 649° C. Compared to lithium, it is easy to handle, machine and dispose. Because of its greater relative abundance, it is lower in cost as a raw material than lithium and magnesium compounds are generally of lower toxicity than lithium compounds. All of these properties coupled with magnesium's reduced sensitivity to air and moisture compared to lithium, combine to make magnesium an attractive alternative to lithium as an anode material.
Sulfur is an attractive cathode material due to its ready availability, low cost, relative nontoxicity and low equivalent weight. Additionally, sulfur has a theoretical maximum capacity of 1675 mAh/g. Therefore, sulfur used as a cathodic material in combination with a magnesium anode could provide a high capacity, safe and economic battery, potentially suitable for use in EV.
Production of a battery having an anode based on magnesium as the active material and a cathode based on sulfur as the active material, requires an electrolyte system which will efficiently transport magnesium ions and which will not adversely affect a sulfur cathode active material. In performance as a cathode active material sulfur is reduced to sulfide and polysulfide discharge products. These discharge products must remain available for oxidation during a charging stage. Moreover, to obtain a viable magnesium sulfur battery, an effective Mg electrolyte transport system cannot be chemically reactive to sulfur. Additionally, in consideration of production of commercial batteries, an electrolyte that can be safely stored, transported and handled is desired.
The electrochemical behavior of a magnesium electrode in a polar aprotic electrolyte solution was reported by Lu et al. in the Journal of Electroanalytical Chemistry (466 (1999) pp 203-217). These authors concluded that the electrochemical behavior of Mg is different from that of Li in polar aprotic electrolyte solutions. Their investigation showed that in contrast to the case of lithium electrodes, surface films which form on the Mg electrode in the aprotic solvents do not transport Mg ions. Therefore, conventional electrolyte systems employed in lithium transport systems are not suitable for a cell having a magnesium anode. Since Mg ion transport is an essential requirement for any electrochemical cell based on a magnesium anode, other electrolyte systems have been investigated.
Gregory et al. (J. Electrochem. Soc., 137 (3), March, 1990, 775-780) reported electrolyte systems of alkylmagnesium halide-organoboron complexes in an ether solvent. Also reported were alkylmagnesium halide solutions to which aluminum halides were added. Mg dissolution and plating at very high current efficiencies, giving bright crystalline Mg deposits were obtained in these systems. However, a suitable cathode material, compatible with the electrolyte system was not reported.
The most commonly used magnesium electrolyte to date is an organometallic material such as phenyl magnesium chloride/aluminum chloride in tetrahydrofuran. However, these electrolyte mixtures are not likely to be of practical commercial utility due to air and moisture sensitivity characteristic of such Grignard-based materials. Moreover, the phenyl magnesium chloride/aluminum chloride electrolyte has limited anodic stability, and significantly, such materials are highly nucleophilic and intrinsically strong reducing agents. This chemical reactivity character is problematic, because to construct an electrochemical cell employing a Grignard type electrolyte, a cathode material which is essentially chemically inert to the Grignard is required. The number of cathode functional materials meeting this requirement are limited. To date there have been two demonstrated cathodes which are compatible with organometallic electrolytes.
Aurbach et al. (NATURE, 407, Oct. 12, 2000, 724-726) describes an Mg battery system containing a magnesium organohaloaluminate salt in tetrahydrofuran (THF) or a polyether of the glyme type as electrolyte and a MgxMo3S4 cathode based on a Mo3S4 Chevrel phase host material. A similar cathode material described as having a formula Mg(0-2)Mo6S(8-n)Sen was also reported by Aurbach (Advanced Materials, 19, 2007, 4260-4267).
Yamamoto et al. (JP2007-233134) describe positive electrode active substances containing fluoro graphite or an oxide or halide of a metal element such as scandium, titanium, vanadium, chromium, manganese iron, cobalt, nickel, copper and zinc. The experimental examples are based on MnO2.
However, the organometallic electrolytes employed in the above magnesium electrolyte systems are highly reactive with sulfur and are known to directly react with sulfur to form sulfides by nucleophilic attack (The Chemistry of the Thiol Group, Pt 1; Wiley, New York, 1974, pp 211-215). Therefore, in order to produce a Mg/S battery, a new electrolyte system which meets all the requirements for magnesium ion transport described previously while having low or no chemical reactivity toward sulfur is required.
U.S. Pre-Grant Publication No. 2009/0226809 to Vu et al. describes a cathode for a lithium-sulfur battery (Abstract). A metal oxide selected from Group I and II metals is included in the composition of a sulfur cathode composition [0012]. The anode contains lithium and the electrolyte described is composed of a lithium salt in a nonaqueous solvent system [0032].
U.S. Pre-Grant Publication No. 2008/0182176 to Aurbach et al. describes an electrochemical cell having a magnesium anode and an intercalation cathode having a modified Chevrel phase. The Chevrel phase compound is represented by the formula Mo6S8-ySey (y is greater than 0 and less than 2) or MxMo6S8 (x is greater than 0 and less than 2). The electrolyte is represented by the formula Mg(AlRxCl4-x)2 and/or (MgR2)x—(AlCl3-nRn)y wherein R is methyl, ethyl, butyl, phenyl and derivatives thereof, n is greater than 0 and lower than 3, x is greater than 0 and lower than 3 and y is greater than 1 and lower than (Claim 3) in an ether solvent.
U.S. Pat. No. 7,316,868 to Gorkovenko describes an electrochemical cell having a lithium anode, a cathode of an electroactive sulfur containing composition and a nonaqueous electrolyte containing a lithium salt and a solvent mixture of dioxolane and one or more of 1,2-dialkoxyalkanes of 5 or 6 carbons and 1,3-dialkoxyalkanes of 5 or 6 carbon atoms (Claim 1). Electroactive sulfur compounds include elemental sulfur and organic compounds having sulfur and carbon atoms (Col. 4, lines 10-26).
U.S. Pat. No. 7,189,477 to Mikhaylik describes an electrochemical cell having a lithium anode, a cathode of a sulfur containing material and an electrolyte system composed of a lithium salt (Col. 4, lines 5-22) and a non-aqueous oxygen containing organic solvent selected from acyclic ethers, cyclic ethers, polyethers and sulfones.
U.S. Pat. No. 7,029,796 to Choi et al. describes a lithium sulfur battery having a cathode of an agglomerated complex of sulfur and conductive agent particles (Claim 1). A solid or liquid electrolyte can be employed and a liquid electrolyte is a nonaqueous organic solvent and a lithium salt (Col. 8, lines 43-58).
U.S. Pat. No. 6,733,924 to Skotheim et al. describes lithium sulfur battery wherein the lithium is protected by a surface coating of a metal such as copper, magnesium, aluminum, silver, etc. (Col. 12, lines 25-28). The electrolyte may be comprised of ionic salts in a non-aqueous solvent, gel polymer or polymer. Ionic electrolyte salts are lithium salts (Col. 15, line 26 to Col. 16, line 27).
U.S. Pat. No. 6,420,067 to Yoshioka describes a hydrogen storage negative electrode being a Mg alloy of Ni, Zn, and Zr (Abstract). The positive electrode is composed of a metal oxide (Col. 3, lines 17-19) and an aqueous electrolyte Col. 7, lines 16-18).
U.S. Pat. No. 6,265,109 to Yamamoto et al. describes air batteries with a negative electrode of a magnesium alloy (Col. 4, lines 9-33). The electrolyte is composed of an acid amide and a second solvent selected from dimethyl acetoamide, acetonitrile, ethylene carbonate, propylene carbonate and γ-butyrolactam (Col. 3, lines 1-15) and magnesium salt of a halogenide or a perchlorate.
U.S. Pat. No. 5,506,072 to Griffin et al. describes a battery having a cathode of finely divided sulfur and finely divided graphite packed about a solid graphite electrode (Col. 3, lines 48-51), an anode containing magnesium and an electrolyte of a corresponding magnesium halide and ionic sulfide as an aqueous electrolyte solution (Col. 3, line 65-Col. 4, line 1).
U.S. Pat. No. 4,020,242 to Okazaki et al. describes a primary cell containing a spacer which contains electrolyte and reduces its apparent volume when pressure is applied by volume increase of the cathode or anode (Abstract). A cell composed of a lithium anode and a cathode of carbon fluoride, silver chromate, manganese dioxide, cupric oxide or vanadium pentoxide and a nonaqueous electrolyte is described (Claim 15).
U.S. Pat. No. 3,849,868 to Jost describes a battery having a container of a composite metal laminate having a layer of magnesium bonded to a laminate material (Abstract). A graphite rod serves as the cathode (Col. 4, line 66 to Col. 5, line 3) and an electrolyte mixture contains manganese dioxide, finely divided carbon and a chromate in an aqueous solution of a bromide salt (Col. 4, lines 48-59).
U.S. Pat. No. 3,658,592 to Dey describes an electric cell having an anode of a light metal (Col. 1, lines 63-67), a cathode of a metal chromate (Col. 1, lines 68-72) and a non-aqueous electrolyte containing inorganic salts of light metals in organic solvents (Col. 1, line 73 to Col. 2, line 9). Magnesium is listed as a light metal.
JP 2004-259650 to Fumihito describes a battery having a magnesium anode and an intercalation cathode of a transition metal (Abstract). A cathode of vanadium pentoxide and graphite is described in Example 1. The electrolyte is a polymer gel containing a phenyl magnesium halide in tetrahydrofuran.
JP 2004-265675 to Hideyuki et al. describes a test cell constructed with a sulfur containing anode and a negative electrode of magnesium metal. Magnesium bis(trifluoromethylsulfonyl)imide in γ-butyrolactone is employed as an electrolyte system.
Aurbach et al. (The Chemical Record, Vol. 3, 61-73 (2003)) describes a study of a (Bu2Mgx)—(AlCl2Et)y system in tetrahydrofuran and reports isolation of single crystals which were identified by XRD analysis as having the following structure:

Aurbach reports that the isolated crystals were inactive with regard to magnesium deposition and describes that Mg could be reversibly deposited from the residual solutions remaining after removal of the precipitated crystals.
In an earlier report of this work (Journal of the Electrochemical Society, 149 (2) A115-A121 (2002)) Aubach reported that the isolated crystals when redissolved in tetrahydrofuran and fresh dibutyl magnesium in hexane added, Mg could be electrochemically deposited and dissolved.
From a practical point of view, with regard to production and use of a commercial battery, electrolyte systems containing agents such as dibutyl magnesium are problematic because of corrosivity, inhalation hazard, flammability and moisture sensitivity.
None of the above documents discloses a practically functional electro-chemical device having an anode containing a magnesium, a cathode containing sulfur and a stable and safe electrolyte system which is effective for the transport of Mg ions and compatible both with an Mg containing active material electrode and also a sulfur containing active material electrode.